The input “zero” adjustment is effectively a process variable offset to compensate for errors in the displayed process variable at the lower part of the input span, but it does affect the readings equally across the entire input span, raising or lowering the apparent value by the value entered. If after adjustment, errors are seen toward the upper portion of the input span, full scale calibration will also be required.
Zero calibration should be carried out towards the bottom of input range. A know signal is applied to the input, and the displayed reading adjusted to match. If the full range of the input isn’t needed for the application, calibration can be carried out at the lowest required value. This will improve the accuracy over the span used, but below this level, the accuracy will be impaired, and may even be outside of the instrument specification.
Caution : Use of accurate reference signals is vital for correct calibration of the input.